1. Field of the Invention
The present invention relates to a method and apparatus for laser welding a pair of metal sheets. More particularly, the present invention relates to a method and apparatus for in-situ laser welding of sheets of material which are to be joined with a hemmed joint. The method and apparatus has particular applicability for assembling parts such as a door of a motor vehicle, wherein inner and outer panels of the door are joined about their perimeters by a welded hemmed joint.
2. Description of the Related Art
It is a common practice in the manufacture of motor vehicle body parts to join sheets of material by hemmed joints, also called hems. Often, hemmed joints are not secured other than by the pressure of the hemming process itself and by adhesive. The adhesive helps to secure the inner and outer panels together and also serves as a corrosion preventative sealer.
It is also known to spot weld locations along the hemmed joints after the stamping process to further secure the connection. However, this is disadvantageous in that it typically requires subsequent metal finishing at the spot weld locations. Additionally, the metal panels may not retain proper orientation to one another when they are transported from the hemming machine to the welding machine.
It also is known to laser weld the hemmed joint after removal from the hemming press. Laser welding has many advantages over conventional resistance welding. For example, laserwelding provides low distortion, small heat affected zones (HAZ), improved mechanical/structural performance, increased speed of processing, improved process controllability/consistency and single sided access. Typically, laser welding systems utilize welding clamps which clamp and fix the parts to be welded together. Robots are used to perform the laser welding operation.
In laser welding, a laser beam generator or source generates or produces a laser beam. The laser beam is transmitted to a focusing head via a fiber optic cable. The focusing head is typically installed or mounted on the end of the robot arm. The robot is then programmed to accurately position the focusing head, and thus the laser beam focal point, relative to the parts to be welded. The welding is done sequentially as the robot moves from weld joint to weld joint. One significant disadvantage of laser welding is the requirement of tight air gaps between the parts to be welded, which in turn requires precise positioning of the laser beam relative to the parts to be welded.
Welding systems for vehicle bodies and the like are described in U.S. Pat. Nos. 4,654,505 issued Mar. 31, 1987 to Sciaky et al. and 5,616,261 issued Apr. 1, 1997 to Forrest. The '505 patent employs a laser beam delivery system using multiple motor driven optical mirrors controlled by a computer program to focus a laser beam upon a single point on the parts to be welded. The '261 patent discloses a laser welding system for welding sheets of material, such as automobile body parts. The system includes a fiber optic cable for each of a plurality of clamps having a first end coupled to a laser beam source and a second end coupled to a clamp for transmitting a laser beam from the laser beam source to the clamp. The system further includes a focusing head secured to each clamp and the second end of the cable for focusing the laser beam onto the material to be welded. U.S. Pat. Nos. 4,654,505 and 5,616,261 are incorporated by reference as if fully set forth herein.
Heretofore, laser welding of hemmed joints has been limited to post-hemming operation in a separate fixture. This approach has various difficulties and limitations, including but not limited to the following: (1) difficulty in beam focus point positioning relative to the critical features of the hemmed flange (i.e., surface, edge, or gap); (2) increased demand on quality control of stamping for flange position repeatability (in the case of preprogrammed robot positioning); (3) increased demand on quality control of stamping for a welded control gap (particularly in the case of lap welding) in order to guarantee weld penetration and strength; (4) onerous requirements for high performance seam tracking ability, which is especially critical for thin sheet metal used in vehicle door outer panels (to deal with the case of uncertain flange position); (5) possible relative movement between the parts to be joined during motion of the hemming machine and prior to welding; (6) possible panel distortion during welding due to sequential welding and lower thermal mass of contact surface elements; and (7) excessively long cycle times, due to limited welding speeds, seam tracking dynamic response time, and time spent by robots to travel between weld sites.
Thus, known welding techniques, including but not limited to those discussed above, present various difficulties in the welding of hemmed joints as known prior systems are intended to laser weld sheets of materials at a location and time spaced apart from other joining and/or assembly procedures.